In order to achieve an effective atomization of biodiesel, droplet breakup in electrostatic field and its mechanism were investigated based on mathematic models and experiment. A droplet electrostatic breakup model was founded by adopting Rayleigh instability conditions, then the main factor affecting the breakup of biodiesel and the specific charges under different influential factors were obtained. The droplets size distribution and specific charges under different voltages were measured by a phase Doppler particle analyzer (PDPA) and the Faraday cage method, respectively. Theoretical analysis and experimental data indicate that the Rayleigh breakup occurs due to surface charges (including net and non-excess polarization charges) to meet Rayleigh instability conditions, and simultaneously the droplets deform and break into much finer droplets to maintain Taylor stability due to polarization forces in electrostatic field. According to theoretical analysis, the key factors affecting the droplets breakup include electrification charge, droplets velocity, and temperature, thus heating, increasing jet velocity and specific charges are considerable methods to overcome the difficulty in biodiesel droplets atomization due to the high viscosity of biodiesel. In experiments, the typical value of specific charge is within 10-9~10-8 C/g, which is much lower than the calculated value 10-6 C/g. The reasons of this discrepancy include the consumption of electrostatic energy for droplets breakup, the increasing non-excess polarization charges on the droplets surface, the droplet polarization in electrostatic field, the aerodynamic forces assisting atomization, and some nature characteristics of biodiesel.
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Charged diesel droplet strip breakup was theoretically reseached and verified by existent experimental results. The main factors affecting the droplet strip breakup were analyzed by adopting Rayleigh limit and considering effect of We and Re Numbers. The critical specific charge of droplets was calculated and its order of magnitude of 10-6~10-5 C/g was obtained. In the droplet range of experiments, the order of magnitude of experimental charge-to-mass ratio was 10-6 C/g. Results indicate that the diesel droplet carrying net charge or polarized charge breakups into more fine drops with strip breakup model in Rayleigh mode and Taylor mode. The experimental results and the calculated data are quite close. This study can provide important foundation for realization of the fuel droplet low pressure atomization. When droplets breaking up, diesel droplet number density dramatically increases and the finer droplets coalesce, causing Sauter mean diameter (SMD) of droplets arriving to peak value. So the relation between the droplet size and charged voltage exhibits non-monotony.
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